Prove that there is a positive integer number n such that the decimal representation of the number: k=1∑⌊3n⌋(3kn)8k ends in 2023 digits 8.
Solution
Let f(n)=∑k=1⌊3n⌋(3kn)8k and ω=1 be a third root of the unity. Using the fact that for every integer k≥0: 1+ωk+ω2k={3,0,if 3∣kotherwise, we get that: f(n)+1=k=0∑⌊3n⌋(3kn)2k=31k=0∑n(1+ωk+ω2k)(kn)2k=313n+31(1+2ω)n+31(1+2ω2)n. Now note that 3,1+2ω and 1+2ω2 are the roots of the polynomial: P(x)=(x−3)(x−1−2ω)(x−1−2ω2)=(x−1)3−8=x3−3x2+3x−9 which, in turn, is the characteristic polynomial of the recursive sequence (ai)i≥0: ai+3=3ai+2−3ai+1+9ai for i≥0. Thus, if we set ai=f(i)+1=1 for 0≤i≤2, then f(n)+1=an for every n≥0. Let bi=ai(mod102023). Since gcd(3,102023)=1, any three consecutive terms of the sequence (bi)i≥0 uniquely determine the previous as well as the next term of this sequence. Together with the fact that there are only finitely many residues modulo 102023, we conclude that the sequence (bi)i≥0 is periodic with some period d>3 (since b3=a3=9). Therefore: 9(f(d−1)+1)=9ad−1=ad+2−3ad+1+3ad≡a2−3a1+3a0(mod102023)=1(mod102023). Finally, since 9∣8.102023+1, we conclude that 998.102023+1≡1(mod102023) and consequently: f(d−1)+1=ad−1≡98.102023+1=202288…89(mod102023)
and thus f(d−1)≡202288…8(mod 102023). Therefore n=d−1 has the desired property. □
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